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[JAVA] setDefaultUncaughtExceptionHandler to log uncaught exception in user thread. #4798

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May 16, 2019
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[JAVA] setDefaultUncaughtExceptionHandler to log uncaught exception in user thread. #4798

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66b6561
Add WorkerUncaughtExceptionHandler
guoyuhong May 16, 2019
4870c10
Fix
guoyuhong May 16, 2019
c3388be
revert bazel and pom
guoyuhong May 16, 2019
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2 changes: 2 additions & 0 deletions java/runtime/src/main/java/org/ray/runtime/runner/worker/DefaultWorker.java
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,7 @@

import org.ray.api.Ray;
import org.ray.runtime.AbstractRayRuntime;
import org.ray.runtime.util.WorkerUncaughtExceptionHandler;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

Expand All @@ -15,6 +16,7 @@ public class DefaultWorker {
public static void main(String[] args) {
try {
System.setProperty("ray.worker.mode", "WORKER");
Thread.setDefaultUncaughtExceptionHandler(new WorkerUncaughtExceptionHandler());
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I don't think It's a good idea to catch all exceptions. Why not simply redirect stderr to the log file?

I guess the process will not crash even if there's a runtime exception in main thread.

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The main thread will catch all the exceptions. For exceptions in other threads, user can catch the exception without triggering this hander. This handler will only be triggered when the exception is not caught by user. It may not be a good idea to suppress this exception. Maybe we need to re-throw this exception? After re-throwing, the error message may still appear in raylet.err. If we redirect the stderr, can this message be redirected to the log4j log file? Can threads write the same file at the same time?

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@raulchen raulchen May 16, 2019 •
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  1. Using LOGGER.error can ensure consistent log format.
  2. This doesn't impact main thread, only the background threads.
  3. If we don't set this, the process won't crash either, because we catch all exceptions in Worker.java

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@raulchen One question, after using setDefaultUncaughtExceptionHandler , will Code snippet 2 be executed in the following code example in another thread? If it is yes, the loggic maybe strange. However, I think it probably won't be executed.

Code snippet 1
throw new Exception();
Code snippet 2

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I guess it won't execute.

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I tried the following code:

@Test
  public void testMultithreadException() {
    Thread.setDefaultUncaughtExceptionHandler(new WorkerUncaughtExceptionHandler());
    Thread thread = new Thread(() -> {
      System.out.println("Before exception.");
      int a = 1/0;
      System.out.println("After exception.");
    });
    thread.start();
    try {
      thread.join();
    } catch (Exception e) {
      System.out.println(e);
    }
  }

The output is:

2019-05-16 17:31:37 INFO RayNativeRuntime [main]: RayNativeRuntime started with store /tmp/ray/sockets/object_store, raylet /tmp/ray/sockets/raylet
Before exception.
2019-05-16 17:31:37 ERROR WorkerUncaughtExceptionHandler [Thread-4]: Uncaught exception in thread Thread[Thread-4,5,main]: {}
java.lang.ArithmeticException: / by zero
	at org.ray.api.test.MultiThreadingTest.lambda$testMultithreadException$0(MultiThreadingTest.java:53)
	at java.lang.Thread.run(Thread.java:748)

===============================================
Default Suite
Total tests run: 1, Failures: 0, Skips: 0
===============================================

Therefore, I think the exception logic won't be break. It is fine to use this handler.

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Maybe just use a lambda here? As the handler is pretty simple.

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This is not a callback function, can this be a lambda?

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Yes, all interfaces with @FunctionalInterface can be represented as a lambda

Ray.init();
LOGGER.info("Worker started.");
((AbstractRayRuntime)Ray.internal()).loop();
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15 changes: 15 additions & 0 deletions java/runtime/src/main/java/org/ray/runtime/util/WorkerUncaughtExceptionHandler.java
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Original file line number Diff line number Diff line change
@@ -0,0 +1,15 @@
package org.ray.runtime.util;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class WorkerUncaughtExceptionHandler implements Thread.UncaughtExceptionHandler {

private static final Logger LOGGER =
LoggerFactory.getLogger(WorkerUncaughtExceptionHandler.class);

@Override
public void uncaughtException(Thread thread, Throwable ex) {
LOGGER.error("Uncaught exception in thread {}: {}", thread, ex);
}
}